Components such as engine casings and other containers intended for use under pressure and made of a composite material having a polymeric matrix reinforced with fibres, are known, in particular in the aeronautical field. Typically, it is known that a manufacturing process based on the winding of fibres or filaments of fibres pre-impregnated with an appropriate polymeric formulation is used for said purpose. An analogous technology has been successfully used for the manufacture of tanks, pipe elements etc. having reduced weight.
Historically, glass fibres have been the most widely used fibres, but more recently other fibres, such as carbon filaments, boron filaments, and high modulus polymeric filaments, in particular aramidic, have been increasingly used in composite structures, thus advantageously exploiting their specific physical-chemical properties.
The resins most often used are typically epoxy-based resins and comprise bisphenol A diglycidyl ethers (BADGE), epoxy diluents with low molecular weight and cross-linking agents such as aliphatic and aromatic amines and anhydrides of carboxylic acids.
Similar polymeric compositions are known, e.g. from WO2009/055666 and U.S. Pat. No. 5,340,946, which disclose formulations comprising an epoxy resin, a blocked isocyanate, a polymer comprising hydroxyl functional groups and a latent cross-linking agent, in particular for the production of printed circuit boards.
In greater detail, WO2009/055666 is particularly concerned with providing polymeric compositions which display a good balance of properties and, in particular, a satisfactory dimensional stability upon heating (i.e. with a sufficiently high glass transition temperature) so that interaction with solders for the manufacture of circuit boards does not cause thermal expansion of the polymeric laminate in the direction perpendicular to the main plane in which the circuit shall lie.
Similarly, U.S. Pat. No. 5,340,946 is concerned with providing polymeric formulations for the manufacture of printed circuit boards which have improved temperature resistance. Even more particularly, formulations are provided which attempt to eliminate the so-called “swimming” of wire-conductors laid down in the resin prior to completion of the curing thereof.
However, printed circuit boards are not subjected, in use, to significant mechanical loads.
Therefore, WO2009/055666 and U.S. Pat. No. 5,340,946 provide no insight on other issues typically encountered when manufacturing composite components of the type comprising fibres embedded in a polymeric matrix, and which are, when in operation, required to withstand much greater loads and stresses, e.g. engine casings, tanks and other containers intended for use under pressure.
Different manufacturing processes are known in the art which entail the winding of a filament around a rotating spindle, the form of which determines the final geometry of the article produced, made of composite material.
According to a first process, called “wet winding”, the fibre filaments are coated with the polymeric formulation contained by immersion in a bath containing the formulation. Immediately after the immersion, the filaments are wound around the spindle to form the desired structure. The structure thus obtained by winding subsequently undergoes cross-linking.
Alternatively, in a second process called “dry winding”, prepregs, or semi-finished products consisting of fibre filaments previously impregnated in the polymeric formulation are wound around the spindle. Also in this case, the structure obtained by means of the winding subsequently undergoes cross-linking. However, for implementation of this second process, it is necessary for the prepreg to have a given stability, i.e. between manufacture and subsequent winding to produce the article made of composite material, it must be possible to easily store the prepreg in a warehouse for a reasonably long period. In other words, the prepreg must have a sufficiently long shelf life also when it is stored at room temperature (so called “out-life”).
In particular, the need is felt in the field for a polymeric formulation for the manufacture of prepregs and articles made of composite material which have a shelf life of at least 5-6 months.
At the same time, polymeric formulations for the manufacture of prepregs and articles made of composite material that possess rheological characteristics fully compatible with the different phases of the manufacturing process and the relative operating conditions (temperature, activation of the cross-linking process, etc.) are highly desirable.
In fact, it must be taken into account that, in the different phases of the process, it is desirable for the rheological behaviour of the polymeric formulation to be progressively adjustable. In particular, in a first phase, the formulation should be sufficiently fluid to wet the fibre filaments, but also sufficiently viscous for the prepreg to be stored stably for a long time before being wound.
In this sense, the need is felt in the sector for a polymeric formulation for the manufacture of prepregs and articles made of composite material which have a higher glass transition temperature than the known formulations.
Furthermore, to ensure workability in the winding phase, it is desirable for the viscosity of the formulation to increase during the winding to a sufficient extent to permit interruption of the spindle rotation. Lastly, it must be possible to cross-link the formulation at the end of the winding phase.
The need is therefore felt in the sector for a polymeric formulation for the manufacture of prepregs and articles made of composite material, the rheological behaviour of which can be adjusted according to the requirements of the production process summarily described above.
Further to the needs mentioned above, in order to improve the mechanical performances of the cured composite and, at the same time, minimize the amount of processing scrap (in terms of fully manufactured components that need to be discarded for failure to comply with certain production requirements), the need is felt in the field for polymeric formulations capable of providing adequate elongation at break values.
Good values of elongation at break of the polymeric formulation would make it possible to obtain a good coefficient of transfer of loads from the polymeric matrix (epoxy resin) to the embedded fibres in the cured composite. This is particularly desirable and relevant in those cases where even small defects (such as the presence of cavities or delamination phenomena) in the cured components,—defects which are, however, substantially inherent with the filament winding technology—would cause the load to be transferred only to the layers with no defects and not to adjacent defective layers, in consequence of the insufficient deformation of the polymeric matrix with respect to the reinforcing fibres. This non-uniform distribution of loads can, as shall be understood, cause damages and significantly reduce the life of the component, which is likely to be more prone to quick deterioration.